Imaging arrays have sensitivities that are limited by 1/f noise caused by gain fluctuations that produce an output drift over time. FIG. 1 shows a graph for a typical 1/f noise spectrum. The 1/f noise has a frequency spectrum (noise vs. frequency (f)) that generally follows a 1/f curve 11 and hence the name for 1/f noise. Above a knee frequency 15 the noise is generally white noise. The cause of 1/f noise is related to properties inherent in all semiconductors, which are used in many applications including imaging arrays. The noise at frequencies below the knee frequency 15 causes the imaging array's output to drift in time. Therefore, it cannot be determined whether the output of a sensor in an imaging array is changing because the scene is changing or whether the output change is due to 1/f noise, unless some step is taken to compensate or calibrate out the drift.
There are currently many methods of calibrating the drift, and these methods can be broken down into two categories: one that applies only to mechanically scanned arrays and one that applies to scanning or staring (non-scanning) arrays.
In mechanically scanned arrays the sensors are moved to scan an image. For example, a mechanically scanned array can be a line array of sensors. Mechanically scanning the imaging elements modulates the signals by creating a time varying element output as the element scans across a scene.
This modulation shifts the image signal to a higher frequency and effectively separates the signal from the 1/f noise in frequencies below the knee frequency. One can subtract the average value of the signal across the entire scan from the scan signal and limit the drift to what occurs within that scan as disclosed by M. A. Janssen, D. Scott, M. White, M. D. Seiffert, C. R. Lawrence, K. M. Gorski, M. Dragovan, T. Gaier, K. Ganga, S. Gulkis, A. E. Lange, S. M. Levin, P. M. Lubin, P. Meinhold, A. C. S. Readhead, P. L. Richards, J. E. Ruhl, “Direct images of the CMB from space,” Astrophysical journal, 1996, pp. 15. This method has the advantage of not requiring any additional hardware; however, appreciable drift can still occur within the scan period. To ensure minimal impact of drift on the sensor performance, the image must be scanned at a rate at least four times the knee frequency, which modulates the image signal to be within the white noise spectrum of the 1/f noise. Because typical commercial sensors have knee frequencies of 1 KHz or more, this method cannot be effectively applied due to the high scan rates required.
The methods used to calibrate staring arrays do not depend on movement of the sensor elements; however, these methods can also be applied to scanned arrays if desired. One method uses a switch, called a Dicke switch, to modulate the image signal, as disclosed in Ulaby, Microwave Remote Sensing, Vol 1, Artech House, Mass., 1981, section 6-9. Another method of modulating the image signal is to use a rotating optical blade, which is called an optical chopper, in front of the sensors. The Dicke switch and the optical chopper both modulate the input signal to move the image signal spectral energy away from the low frequency noise, thereby minimizing drift effects.
The Dicke switch must be installed in each element separately, and therefore adds significant cost to the array. Furthermore, the Dicke switch introduces losses that degrade the sensitivity of the array.
An optical chopper has the advantage of modulating all of the elements at once because it can be placed in front of all the sensors. The drawback of optical choppers is that they cannot spin at high enough rates to modulate the image signal above typical knee frequencies. In addition, optical choppers often create audible noise and also require significant space when used with large arrays. Because an optical chopper is a moving part, more maintenance is required.
Another method of drift compensation is called noise injection. In this scheme each sensor contains a noise source that is coupled into each sensor input. The noise source is switched on and off at a rate higher than the knee frequency. By taking the ratio of the output of the sensor during the on and off times, one can eliminate the output drift due to temporal gain fluctuations. This method is disclosed in Ulaby, Microwave Remote Sensing, Vol 1, Artech House, Mass., 1981, section 6-12. John D. Kraus, in Radio-Telescope Receivers, McGraw Hill, N.Y., 1966, pages 289-290 discusses the same method for a radio telescope receiver. This method requires additional hardware to be designed into each of the sensors, adding significant cost. Furthermore, the ability to calibrate out drift is limited to the inherent stability of the noise source. Noise sources contain uncontrolled amplitude fluctuations, typically with a 1/f type of noise spectrum, and these fluctuations add additional drift to the output that cannot be compensated using the noise injection method disclosed by Ulaby and Kraus.
What is needed is a method for compensating out 1/f noise for an arbitrarily sized array of sensors, whether the sensors are mechanically scanned or staring. Also needed is a method that is not limited by the stability of the output of a reference or noise source. Also each individual sensor in the imaging array should be able to be compensated while adding only a small cost to the imaging array. The embodiments of the present disclosure answer these and other needs.